Nickel-chromium-iron alloys



Aug. 31, 1965 A. w. FRANKLIN ETAL NIGKEL-CHROMIUM-IRON ALLOYS Filed Aug. 10, 1962 INVENTORS Awwu/y M fZ A Yku y BY Few/41.014. firm-H 50/4/41 0 G. Flo/M4 06 ATTOP/YEY United States Patent 3,203,791 [NICKEL-CIROMIUM-RUN A-IJLOYS Arthur William Franklin, Quinton, Ronald Alfred Smith,

West Hagley, and Edward Gordon Richards, Small Heath, England, assignors to The International Nickel Company, Inc New York, N.Y., a corporation of Delaware Filed Aug. 10, 1962, Ser. No. 216,125 Claims priority, application} *Gnealt Britain, Aug. 11, 1961, 2 claims. (Cl. 75-171) The present application is a continuation-impart of our copending patent application Serial No. 95,740, filed March 14, 1961, now Patent No. 3,094,414.

The present invention relates to nickel-chromium alloys and, more particularly, to weldable nickel-chromium alloys including nickel-chromium-iron alloys.

Nickel-chromium and nickel-chromium-i-ron base heatand creep-resistant alloys containing titanium and aluminum to provide a precipitable phase of Ni (Ti, Al) type and also containing molybdenum are now well known. In most alloys of this type, however, it is found that the ductility decreases with increasing temperature and is generally at a minimum in the temperature range 700 C.- 850 C. While these alloys in the form of thin sheet not more than about /s inch thick can be welded under mild conditions and without restraint, the ductility of the Welded joints decreases to an even greater extent in this temperature range so that the elongation of the welded joints in high temperature tensile tests may fall below the commercially desirable minimum of 5% or 7%. To overcome this loss of ductility it has hitherto been necessary to apply high temperature heat treatments after welding. The difiiculty in fabricating components by welding is particularly acute when the alloys are in sheet form and are used to make components such as jet-pipes for aircraft gas turbines, since at the high temperatures that must be used for the heat treatments, the components tends to collapse or distort. Moreover, it is often difiicult or impossible to apply a post-weld heat treatment when the components are repaired in service under conditions Where large scale heat treatment facilities are not available. Although attempts were made to overcome the foregoing difficulties and other disadvantages, none, as far. as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that by means of a specially restricted alloy composition it is now possible to rovide alloy structural elements which are readily weldable.

It is an object of the present invention to provide a novel weldable alloy.

Another object of the invention is to provide a novel welding process.

The invention also contemplates providing novel welded structures.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

The figure is a graph illustrating the relationship be-- tween titanium and aluminum contents which are characteristic of alloys in accordance with the present invention.

Generally speaking, the present invention contemplates providing heatand creep-resistant alloys that largely retain their strength and ductility after welding with a simple or no post-weld heat treatment. This object is achieved by adding critical amounts of the elements carbon, titanium, aluminum, molybdenum, boron and zirconium to nickel-chromium or nickel-chromium-iron base alloys containing about and especially about 16% to about 17% chromium.

Patented Aug. 31, 1965 ice Alloys according to the invention contain, in percent by weight, chromium about 15% to 25% and preferably about 16% to about 17%, iron 0% to 45%, carbon about 0.04% to 0.15%, titanium about 0.7% to 2.5%, aluminum about 0.7% to 1.5%, molybdenum about 3% to 6% and preferably 3.5% to 4.5%, boron 0.001% to 0.009%, zirconium 0.01% to 0.1%, silicon 0% to about 0.5% manganese 0 to about 0.5%, cobalt 0% to about 1%, the balance (apart from impurities in an amount not exceeding 0.5%) being nickel in an amount not less than 35%. In addition, the total titanium and aluminum content is 2% to 3.5%, the ratio titanium/aluminum is 0.5 to 4 and the Ti+Al content is so correlated with the Ti/Al ratio that the alloy lies within the area ABCDA in the figure of the accompanying drawing.

It is found that within these preferred ranges of composition little or no formation of sigma phase occurs on welding or on exposure in service to temperatures of 600 C.800 C. for times up to about 1000 hours.

The exact proportions of each of the added elements is of great importance in achieving satisfiactory roperties in the welded alloy.

The carbon content should be at least 0.04% in order to obtain an acceptable level of ductility. However, at carbon contents higher than 0.15%, free carbides are formed in the alloy during working that reduce the creep resistance at elevated temperatures and also make it difiicult to form the alloy into sheet or shape the sheet into components. Prolonged exposure to temperatures above 600 C. leads to precipitation of carbides at carbon contents of less than 0.16% and at carbon contents of above 0.08%, these carbides tend to be deposited at the grain boundaries in a form that may ultimately lead to embrittlement of the alloys, after say 500 hours or more. This efiect is particularly pronounced at temperatures of less than 700 C.

It is important that alloys for use under conditions of stress at high temperatures should not only be resistant to creep but also should not become embrittled on prolonged exposure to high temperatures. On the other hand, for short time applications in which the greatest ductility is required, the carbon contents up to 0.12% may advantageously be used.

Both the total content of titanium and aluminum and the Ti/Al ratio are important. The effect of increasing the Ti+Al content is to increase the strength of the alloy and at the same time decrease its tensile ductility, both at room temperature and at elevated temperatures. At Ti+Al contents below 2% the strength is inadequate, while above a Ti+Al content depending on the Ti/Al ratio and defined by the line A-B in the figure of the drawing, the alloys have very low ductilities at elevated temperatures, particularly after welding. Alloys having Ti+Al contents corresponding to points substantially above the line A-B but less than 3.5 have undesirably low ductility at high temperatures after welding and at Ti+Al content-s greater than 3.5%, their room temperature ductility also rapidly falls off to such an extent that fabrication of the alloy in the form of sheets becomes impractical. For applications where the greatest ductility is required and strength is of rather less importance, it is advantageous to make use of alloys within the area EFCDE. The optimum combination of strength and ductility is obtained within the area ABFEA at Ti+Al contents below 3% while if the highest strength is required at the expense of some sacrifice in ductility, alloys within the portion of the area ABFEA that corresponds to Ti+Al contents above 3 should be used.

Replacement of nickel by iron results in some decrease in the tensile strength at high temperatures and alloys with a low iron content, for example, 0% to 5% of iron,

are therefore advantageous for applications where the highest tensile strengths are required. Alloys with say 30% to 40% iron, on the other hand, can be used where the highest ductilities are required but the tensile strength is not of such great importance.

is exhibited by alloys in accordance with the present invention which have a carbon content about 0.04% to about 0.08%, a chromium content about 16% to about 17%, a molybdenum content about 3% to about 3.5%, and a nickel content about 37% to about 50%, the re- 5 It is important that the nickel content should not be mainder of the composition being within the ranges set reduced below 35%, other-wise severe embrittlement ocforth hereinbefore. Advantageously, the titanium concurs after long exposure to temperatures in the range 650 tent is about 1.0% to about 2.5% and the aluminum con- C.-850 C. tent is about 0.7% to about 1.5%. In the case of alloys The presence of molybdenum improves the creep rein which the total content of titanium and aluminum is sistance of the alloys at high temperatures and for this 3.0% or more it is preferred that the nickel content should purpose at least 3% molybdenum should be present. On be at least 42%. For reasons of economy it may, howthe other hand, the molybdenum content must not exceed ever, be desirable to restrict the nickel content to a maxi- 6% as at higher levels the corrosion resistance becomes mum of 45%. seriously impaired and forging is almost impossible. With- By way of example, three alloys were prepared having in the range 3% to 6%, the effect of molybdenum on the the compositions given in Table I:

TABLE I Alloy Per- Per- Per- Per Per- Per- Per- Per- Per- Per- Per- N 0 cent cent cent cent cent cent cent cent cent cent cent Ni Cr Mo 0 Mn Si Ti Al B Zr Fe 1 45.4 16.5 3.0 .009 0. 05 03 1.33 1. 33 .005 .02 Bal. 2 42.7 15.0 3.23 .05 0. 07 .03 2.13 0.91 .001 .05 Bal. a--- 51.5 17.2 5.18 .04 0.07 0.23 2. 06 0.00 .001 .05 Bal.

high temperature strength of the alloys is complementary These alloys were cast into ingots that were forged to to that of Ti-l-Al. At a given Ti+Al content, increasing bar, which was then heat treated by solution heating at the molybdenum content increases the high temperature 1080 C. for 8 hours, air cooling, aging at 700 C. for strength and the fall in strength that occurs on reducing the 16 hours, and again air cooling. Stress-rupture properties Ti-i-Al content from a given level can be partly or wholly determined on specimens of alloy No. 1 machined from compensated by increasing the molybdenum content by the bar are shown in Table II: 0.5% for every 0.1% that the Ti+Al content decreases.

It has been surprisingly found that very small amounts TABLE II of boron improve the tensile ductility of the alloys at high Composition (percent by weight) temperatures and a content of boron w1th1n the very narrow range 0.001% to 0.009% is essential if welded joints formed in the alloys are to have adequate ductility at ele- Alloy Egg tli g fig g gi fig vated temperatures and are to be satlsfactorily hot work- (hour) (percent) able. At boron contents below 0.001%, the ductility of the alloys after welding is low and so is the stress-rupture 1 as 050 57 12.8 life. On the other hand, if the boron content is increased 28 650 948 1&5 above 0.009%, the alloys tend to crack on Welding, particularly in thick sections exceeding say inch, and 1 tolls 0 p unds) per square inch.

preferably it does not exceed 0.004%. suitable content is 0.003%.

Zirconium contributes to the creep resistance of the alloys and for this purpose it should be present in the range 0.01% to 0.1%, preferably 0.05%. Greater amounts of A particularly zirconium than 0.1% again may lead to cracking on welding.

It has now been found that a particularly satisfactory combination of properties, including freedom from embrittlement on prolonged exposure to high temperatures,

The results of tensile and impact tests carried out at 20 C., 550 C., 650 C. and 750 C. on further specimens of alloy No. 1, and of impact tests at 20 C. carried out on specimens of alloys Nos. 2 and 3, are set forth in Table III. All the specimens used were machined from the heattreated bar. The impact strengths were determined immediately on reaching the test temperature and also after exposure to prolonged heating to high temperatures, the temperatures and times of exposure being shown in the third column of the table.

TABLE III Exposure U.T S. Yield Elongation Impact Alloy N0 temperature Hours/C. (t.s.1.) stress (percent) resistance (C.) (t.s.1.) (ft. lb.)

The preferred alloys are particularly suitable as materials for components of steam plant, for example superheater tubes, steam pipes and parts of steam turbines, and for other articles subjected to stress for long .periods of elevated temperatures.

The results in Table III show clearly that both alloys Nos. 1 and 2 had substantial impact strength even after very long exposure to temperatures generally higher and more likely to lead to embrittlement than the service range encountered in steam plant. On the other hand, the impact strength of Alloy No. 3 was substantially lower even after 1000 hours at 750 C.

The alloys may be melted in any convenient manner. It is common practice in air melting nickel-chromium alloys to deoxidize them by an addition of calcium or magnesium. When this technique is used for the present alloys, particlarly those containing less than 5% iron, care should be taken that the residual calcium or magnesium content is a low as possible, preferably not exceeding 0.005%, as otherwise cracking may occur on welding thick sections or under restrained conditions and the high temperature ductility of the welded joints may be impaired. To develop the optimum properties in the alloys, it is essential that they should be given an age hardening heat treatment consisting of solution heating followed by aging at a lower temperature. Advantageously, this treatment consists of heating at 1020 C.-1150 C., followed by cooling in air and aging at 650 C.850 C. For thin sections, say less than 75 inch, the first treatment should be for 2 30 minutes and the second treatment for 2-16 hours. For thicker sections, the first treatment should be for a longer time, say 2-8 hours. All the statements in this specification about the properties of the alloys refer to specimens that have been age hardened in this way. It is to be observed that welding as employed in this specification refers to a process wherein elements, structures and the like are joined by means of a fusion process during which a liquid metallic content is established while at least the adjacent surfaces of the structures, elements, etc., to be joined are at a temperature in excess of the incipient fusion temperature of the alloy or metal from which the structures or elements are made. Subsequent cooling during which contact is maintained establishes the weld bond by freezing of the liquid contacting metal.

If the alloys are to be welded, a solution heating hould be applied before the welding operation is carried out but aging before welding is unnecessary. After welding, no further solution heat treatment is in general necessary but where maximum strength is required, it can be desirable to carry out a post weld aging treatment consisting of heating for 2-16 hours at 650 C. to 850 C. or 900 C. for preference in the higher part of this temperature range.

The molybdenum in the alloys can be wholly or partly replaced by an equal atomic percentage of tungsten.

The alloys are suitable for use both in the sheet and wrought forms and can be used in the manufacture of welded structures, including composite welded structures comprising sheet components with wrought stiffening members welded to them. Examples of such structures are parts of aircraft gas turbines such as jet pipes, flame tubes and jet silencers and steam pipe assemblies for use in contact with superheated steam.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that m-odifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. An alloy characterized by highly enhanced resistance to impact after a long-time exposure of 1000 hours to elevated temperatures up to about 750 C. and consisting essentially of, by weight, about 16% to about 17% chromium, about 0.04% to about 0.08% carbon, about 3% to about 3.5% molybdenum, about 0.7% to about 2.5% titanium, about 0.7% to about 1.5% aluminum, about 0.001% to about 0.009% boron, about 0.01% to about 0.1% zirconium, up to about 0.5% silicon, about 37% to about 50% nickel, up to about 0.5% manganese, up to about 1% :cobalt with the balance being essentially iron, the sum of the said titanium percentage and said aluminum percentage being 2% to about 3.5% and the ratio of said titanium percentage to said aluminum percentage being about 0.5 to about 4 and said sum being correlated with said ratio so that when the values of said sum and said ratio are plotted, the resultant point lies within the area ABCDA set forth in the figure on the accompanying drawing.

2. An alloy characterized by highly enhanced resistance to impact after a long-time exposure of 1000 hours to elevated temperatures up to about 750 C. and consisting essentially of, by weight, about 16% to about 17% chromium, about 0.04% to about 0.08% carbon, about 3% to about 3.5% molybdenum, about 0.7% to about 2.5% titanium, about 0.7% to about 1.5% aluminum, about 0.001% to about 0.009% boron, about 0.01% to about 0.1% zirconium, up to about 0.5% silicon, about 42% to about 45% nickel, up to about 0.5% manganese, up to about 1% cobalt with the balance being essentially iron, the sum of said titanium percentage and said aluminum percentage being 3% to about 3.5 and the ratio of said titanium percentage to said aluminum percentage being about 0.5 to about 4 and said sum being correlated with said ratio so that when the values of said sum and said ratio are plotted, the resultant point lies within the area ABFEA set forth in the figure on the accompanying drawmg.

References Cited by the Examiner UNITED STATES PATENTS 2,920,956 1/ 60 Nisbet et a1 171 2,957,239 10/60 Pritchard et a1 29488 2,977,222 8/ 61 Bieber 7 5171 3,051,565 8/62 Pitler et 'al. 75134 FOREIGN PATENTS 548,776 11/57 Canada. 728,375 4/55 Great Britain.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner. 

1. AN ALLOY CHARACTERIZED BY HIGHLY ENHANCED RESISTANCE TO IMPACT AFTER A LONG-TIME EXPOSURE OF 1000 HOURS TO ELEVATED TEMPERATURES UP TO ABOUT 750*C. AND CONSISTING ESSENTIALLY OF, BY WEIGHT, ABOUT 16% TO ABOUT 17% CHROMIUM, ABOUT 0.04% TO ABOUT 0.08% CARBON, ABOUT 3% TO ABOUT 3.5% MOLYBDENUM, ABOUT 0.7% TO ABOUT 2.5% TITANIUM, ABOUT 0.7% TO ABOUT 1.5% ALUMLINUM, ABOUT 0.001% TO ABOUT 0.009% BORON, ABOUT 0.01% TO ABOUT 0.1% ZIRCONIUM, UP TO ABOUT 0.5% SILICON, ABOUT 37% TO ABOUT 50% NICKEL, UP TO ABOUT 0.5% MANGANESE, UP TO ABOUT 1% COBALT WITH THE BALANCE BEING ESSENTIALLY IRON, THE SUM OF THE SAID TITANIUM PERCENTAGE AND SAID ALUMINUM PERCENTAGE BEING 2% TO ABOUT 3.5% AND THE RATIO OF SAID TITANIUM PERCENTAGE TO SAID ALUMINUM PERCENTAGE BEING ABOUT 0.5 TO ABOUT 4 AND SAID SUM BEING CORRELATED WITH SAID RATIO SO THAT WHEN THE VALUES OF SAID SUM AND SAID RATIO ARE PLOTTED,THE RESULTANT POINT LIES WITHIN THE AREA ABCDA SET FORTH IN THE FIGURE ON THE ACCOMPANYING DRAWING. 